Date: Mon, 10 Dec 2001 14:20:03 -0800 From: Peter Wemm <peter@wemm.org> To: Ian Dowse <iedowse@maths.tcd.ie> Cc: Joerg Wunsch <joerg_wunsch@uriah.heep.sax.de>, freebsd-current@FreeBSD.ORG Subject: Re: cvs commit: src/sys/kern subr_diskmbr.c Message-ID: <20011210222003.4391539F0@overcee.netplex.com.au> In-Reply-To: <200112101150.aa33241@salmon.maths.tcd.ie>
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Ian Dowse wrote:
> In message <20011210004350.6703C3810@overcee.netplex.com.au>, Peter Wemm writ
es
> :
> >The problem is, that you **are** using fdisk tables, you have no choice.
> >DD mode included a *broken* fdisk table that specified an illegal geometry.
> ...
> >This is why it is called dangerous.
>
> BTW, I presume you are aware of the way sysinstall creates DD MBRs;
> it does not use the 50000 sector slice 4 method, but sets up slice
> 1 to cover the entire disk including the MBR, with c/h/s entries
> corresponding to the real start and end of the disk, e.g:
>
> cylinders=3544 heads=191 sectors/track=53 (10123 blks/cyl)
> ...
> The data for partition 1 is:
> sysid 165,(FreeBSD/NetBSD/386BSD)
> start 0, size 35885168 (17522 Meg), flag 80 (active)
> beg: cyl 0/ head 0/ sector 1;
> end: cyl 1023/ head 190/ sector 53
> The data for partition 2 is:
> <UNUSED>
> The data for partition 3 is:
> <UNUSED>
> The data for partition 4 is:
> <UNUSED>
>
> Otherwise the disk layout is the same as disklabel's DD. I suspect
> that this approach is much less illegal than disklabel's MBRs
> although I do remember seeing a HP PC that disliked it. I wonder
> if a reasonable compromise is to make disklabel use this system for
> DD disks instead of the bogus 50000 sector slice? Obviously, it
> should also somehow not install a partition table unless boot1 is
> being used as the MBR, and the fdisk -I method should be preferred.
Yes, that is much safer, however there are certain bioses that have
interesting quirks that the MBR has to work around. The problem when
overlapping mbr and boot1 into the same block is that we no longer have the
space to do that. boot1.s has got *3* bytes free.
For example, we dont have space to fix the case where the drive number is
passed through incorrectly to the mbr. Some older Intel boards have this
problem (Phoenix derived bios). See boot0's setdrv option.
Also (and I think this is more likely to be the problem you ran into) many
newer PC's are looking at the partition tables for a suspend-to-disk
partition or a FAT filesystem with a suspend-to-disk dump file. For better
or worse, PC architecture dictates that boot disk partitions start and end
on cylinder boundaries (except for the first one which starts on the second
head in the first cylinder). When we break those rules, we have to be
prepared for the consequences.
However, there is light at the end of the tunnel. EFI GPT is pretty clean.
It supports up to something like 16384 partitions and it has all the useful
stuff we could possibly want including unique ID's, no CHS (pure 64 bit
LBA), volume tags (you can name partitions etc), and so on. It is clean
enough that we could almost get away with doing away with disklabel as
well. "Coming soon to a PC near you."
(http://developer.intel.com/technology/efi/index.htm)
Cheers,
-Peter
--
Peter Wemm - peter@FreeBSD.org; peter@yahoo-inc.com; peter@netplex.com.au
"All of this is for nothing if we don't go to the stars" - JMS/B5
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